1. Field of the Invention
This invention relates in general to the manufacture of cement clinker in long rotary kilns. In particular, the invention relates to the method and apparatus for the manufacture of cement clinker in conventional long wet or dry rotary kilns wherein steel slag is added at the input-end of kiln with a stream of feedstock material containing lime such that as the stream of feedstock and steel slag moves toward the heat at the heat-end of the kiln, the steel slag is melted and defused into the feedstock material to form cement clinkers.
2. State of the Art
As stated in U.S. Pat. No. 5,156,676, the literature is replete with processes by which the calcining and clinkering of cement ingredients can be accomplished. The typical process using a rotary kiln, either wet or dry, is well known. Cement raw materials such as limestone, clay and sand, or the like, are finely ground and intimately mixed to provide a substantially homogeneous mixture at the input or feed-end of the kiln. The kiln is tipped downwardly at an angle such that the heat-end of the kiln is below the feed-end. The kiln has generally four operating zones including a precalcining zone, a calcining zone, a clinkering zone, and a cooling zone. Conventional fuel is combined with preheated air and injected into the kiln at the heat-end. Fuels such as natural gas, oil or powdered coal are conventionally employed in cement manufacturing processes.
As the finely divided cement raw materials pass into the rotating kiln at the feed-end thereof, the materials are heated from near ambient temperature to about 538.degree. C. (1000.degree. F.) in the precalcining zone. In this zone, the heat of the combustion gases from the calcining zone is used to raise the temperature of the raw materials. Additionally, in the kiln, chain systems or the like may be attached to the interior of the kiln and are employed to improve the efficiency of heat exchange between the gases and raw materials.
The temperature of the raw materials is increased from about 538.degree. C. (1000.degree. F.) to about 1093.degree. C. (2000.degree. F.) as they pass through the calcining zone and in this zone CaCO.sub.3 is decomposed with the evolution of CO.sub.2.
Calcined material at the temperature of about 1093.degree. C. (2000.degree. F.) then passes into the clinkering or burning zone where the temperature is raised to about 1500.degree. C. (2732.degree. F.). It is in this zone that the primary raw materials are converted into the typical cement compounds such as tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium-aluminoferrite. The cement clinkers then leave the clinkering zone where the clinkers are cooled and thereafter processed further such as by grinding.
Further, the use of ground blast-furnace slag as a cementitious material dates back to 1774. In the production of iron, the blast furnace is continuously charged from the top with iron oxide sources, fluxing stone, and fuel. Two products are obtained from the furnace: molten iron that collects in the bottom of the furnace and liquid iron blast-furnace slag floating on the pool of iron. Both are periodically tapped from the furnace at a temperature of about 1500.degree. C. (2732.degree. F.). The slag consists primarily of silica and alumina combined with calcium and magnesium oxides from the fluxing stone. Cementitious activity of this slag for use in mortar or concrete is determined by its composition and the rate at which the molten material is cooled when it comes from the furnace.
Further, in the production of steel, a similar process occurs wherein liquid steel slag floats on the pool of steel. Again, the steel slag consists primarily of silica and alumina combined with calcium and magnesium oxides. Disposing of both the steel slag and the blast-furnace slag poses a major disposal problem for the manufacturer thereof because of the volumes of material involved.
Both the steel slag and the blast-furnace slag is composed of particles that are very hard. The blast-furnace slag, when used, has always been in a finely powdered form, which means that a great deal of energy must be used to grind and pulverize the slag into the finely powdered form. Such a process is disclosed in U.S. Pat. No. 2,600,515 in which a blast-furnace slag, in a finely powdered mixture with limestone, is fed in rotary cement kilns and is introduced directly into the flame of the kiln. The slag powder is blown in at the same time and by the same channels as the fuel, namely pulverized coal, heavy oil or gas. This process has several disadvantages. One of the most significant disadvantages is that enormous amounts of energy are required to pulverize and dry the material so that it could be blown into the furnace.
Many of the chemical compounds in steel slag and blast-furnace slag are common to cement chemical compounds and their heat of formation is already been accomplished in their respective processes. X-ray diffraction analysis of steel slag shows the composition to be a highly fluxed beta (.beta.) dicalcium silicate 2CaO.SiO.sub.2 (C.sub.2). This compound, with the addition of CaO, can be converted to 3CaO.SiO.sub.2 (C.sub.3 S) in the burning zone of the rotary kiln.
Applicants experience has shown steel slag to have no deleterious effect on the operation of a cement rotary kiln. Emission of volatile materials from the rotary kiln is improved because the slag has previously been heat treated and most volatile materials have been removed, i.e. carbon dioxide, carbon, volatile organics, and the like. However, as stated, fine grinding or comminution or pulverization of the slag is required, thus adding an expensive step to the cement-making process.